This invention relates to lubricating oil compositions for use in engines having silver bearings. In a further aspect, the invention relates to the protection of silver bearing parts in internal combustion engines.
As is well known, lubricating oils for heavy duty diesel engines require crankcase lubricating oils which are stabilized against oxidation and which limit the formation of engine deposits. In addition, these crankcase lubricating oils must also have a high alkalinity to neutralize acids formed during fuel combustion.
Many heavy duty railroad and tugboat diesel engines in use in the United States pose an additional lubrication problem, because they have silver-surfaced engine parts, such as silver or silver-plated bearings. While the foregoing properties of oxidation stability, deposit control and alkalinity can be achieved by the use of lubricating oil additives known in the art, many of the resulting oils cause unacceptable corrosion and wear to silver-surfaced diesel engine parts. Silver, or silver-surfaced bearing parts, pose a special problem since many of the bearing protection additives which are effective to protect bearings surfaced with other materials, e.g., brass, copper-lead, bronze, aluminum, are ineffective to protect silver bearing parts or, for example in the case of materials such as zinc dithiophosphate, commonly used in automotive spark-ignition engine lubricants, are known to be deleterious to silver bearings. A further problem is that in the case of railroad and inland marine engines, many have non-silver bearings, e.g., copper-lead bearings. However, pragmatically the user will want to use the same lubricating oil for all engines regardless of whether they have silver bearings or copper-lead bearings. Thus, as well as being effective for silver bearing parts, the lubricating composition should also be effective for non-silver bearings.
At present silver protection is largely provided by the use of lubricants containing chlorinated paraffins or other chlorinated additives. Examples of halogenated additives used to provide silver protection are, for example, described in U.S. Pat. Nos. 4,169,799; 4,171,269; and 4,131,551. However, a problem has arisen with respect to the use of halogenated additives in that they are perceived as presenting environmental problems. Thus, there is a need for lubricants which provide silver protection without the inclusion of halogenated additives.
In view of this need the art has already developed certain halogen-free or reduced halogen silver corrosion inhibitor-containing lubricants. For example, U.S. Pat. Nos. 4,764,296 and 4,734,211 disclose a marine and railway diesel engine lubricating oil composition containing certain polyhydroxy esters as silver wear inhibitors. These patents also disclose lubricating oil compositions containing a mixture of these polyhydroxy esters and chlorinated paraffins. U.S. Pat. No. 4,820,431 discloses a method for reducing silver wear in marine and railway diesel engines using similar lubricating oil compositions. U.S. Pat. No. 4,171,270 discloses lubricating oil compositions containing a sulfurized overbased calcium alkylphenolate and a sulfurized naphthenic base oil-containing composition having a sulfur content of from 1 percent to 6 percent by weight. These compositions are also thought to have silver protective properties. U.S. Pat. No. 4,871,465 discloses lubricating oils containing as a silver protectant (a) a sulfurized olefin, sulfurized fatty acid, sulfurized hydroxyaromatic, sulfur containing heterocyclic compounds, organic sulfide or dithiocarbamate and (b) the reaction product of a saturated aliphatic dicarboxylic acid with an optionally substituted aminoguanidine.
Other organic compounds have also been disclosed as providing silver protection. Thus U.S. Pat. No. 4,278,553 discloses a railway diesel engine lubricant containing a silver corrosion inhibitor comprising a benzotriazole compound present in concentrations from about 0.5 to 2.0 wt % and U.S. Pat. No. 4,285,823 discloses a diesel lubricant composition containing a silver corrosion-inhibiting amount of an N-substituted 5-amino-1H-tetrazole.
A continuing need exists for additives which provide silver protection without presenting potential environmental problems and preferably, which are readily available, relatively inexpensive and which are not deleterious to non-silver bearings such as copper-lead bearings. As before mentioned, a significant problem in meeting this need is the unpredictability of additive response with respect to silver bearing systems. One simply cannot predict whether a given additive will provide silver bearing protection based on its properties in lubricants for non-silver bearing engines or a general characterization of the additives' properties or function. Thus, for example, zinc dithiophosphates which are widely used to provide wear and oxidation protection, are recognized to be deleterious to the silver bearing parts of engines.
The art has suggested the addition of unsaturated carboxylic acids and a variety of esters thereof to various types of lubricants for a variety of reasons. Thus, as early as 1958, U.S. Pat. No. 2,851,422 suggested adding certain sulfurized compounds (e.g., sulfurized sperm oil) to transmission fluids to protect against corrosion and adding oleic acid to such compositions as an anti-squawk agent.
U.S. Pat. No. 2,830,956, directed to hydraulic power transmission fluids, teaches that it is well known that various of the common fatty acids, fatty oils and esters and their chlorinated derivatives have good oiliness characteristics and in accordance with patentee's invention, suggests adding the combination of an oil-soluble fatty acid having 14-22 carbon atoms with sulfurized or unsulfurized fatty oil to transmission fluids to impart improved oiliness. Patentee teaches that both the use of the two oiliness agents and the specific proportions taught by patentee are critical to the invention. Patentee also teaches that oleic acid, stearic acid, erucic acid are objectionally corrosive to certain types of metal but that this drawback can be overcome by the inclusion of conventional corrosion inhibitors and suggests sulfurized olefinic hydrocarbons as satisfactory corrosion inhibitors.
U.S. Pat. No. 3,267,033 directed to additives in lubricants for internal combustion engines, especially power transmission units, describes an additive combination of 1 to 3 parts by wt. of an oil-soluble fatty acid, preferably unsaturated and most preferably oleic acid, and 1 to 3 parts of a tertiary aliphatic primary amine salt of a partially esterified phosphoric acid as imparting desirable friction properties. Patentee also teaches that a particularly effective combination of additives for use in lubricants in gear assembles or differentials consists of the aforementioned additive combination and a hydrocarbon polysulfide and/or a neutralization product of an amine with certain acids.
Canadian Patent No. 1,228,847 is directed to lubricant compositions broadly comprising an aliphatic olefinic compound and a sulfurized olefinic hydrocarbon in a hydrorefined lubrication oil which are described as providing longer lasting properties, anti-wear capability and reduced staining of copper parts. Patentee teaches that the aliphatic olefinic compound is preferably a fatty acid or more preferably a fatty acid ester. The Canadian patent gives various examples of acids including tall oil and those obtained by the hydrolysis of fats such as palmitoleic acid, oleic acid, linoleic acid, linolenic acid, etc. Although primarily directed to multipurpose industrial oils for use in gear, hydraulic and other specialty applications, the patent also broadly teaches that its lubricating compositions can be used for a wide variety of purposes including crankcase lubricants for spark-ignition and compression-ignition combustion engines, including automotive and truck engines, two-cycle engines, aviation piston engines, marine and railroad diesel engines and the like and for stationary power engines, turbines, transmissions, transaxles, metal working lubricants and other lubricating oils and greases.
U.S. Pat. Nos. 3,112,269 and 3,112,271 cursorily teach that esters of carboxylic acids or phosphoric acid, e.g., partial esters of fatty acids and polyhydric alcohols or alkyl phosphites or phosphates, or free fatty acids and sulfuric derivatives thereof such as C.sub.10 -C.sub.18 fatty acids (oleic or stearic acids) and sulfurized unsaturated fatty acids, e.g., sulfurized oleic acid, are anti-wear and extreme pressure agents for hydrocarbon combustions but when used in lubricating oils subject to high temperatures and pressures, break down and fail to impart their expected desired properties under extreme conditions.
U.S. Pat. No. 2,788,326 teaches that improved extreme pressure lubricants can be obtained by the use of heat polymerizable polyfunctional organic compounds containing a plurality of hydroxyl group, e.g., sorbitan monoleate.
U.S. Pat. Nos. 4,780,111 and 4,412,928 refer to the use of terephthalic acid to provide corrosion protection for lead bearings.
U.S. Pat. No. 3,041,284 suggests the use of fatty acids and fatty acid esters as clarifiers in oils containing certain mercapto modified acid or alcohol additives.
U.S. Pat. No. 4,428,850 suggests adding an estolide of a hydroxy fatty acid in railroad diesel engine lubricating oils as an antifoaming agent.
In the early '70's oleic acid was investigated as a friction modifier additive for lubricating oils for passenger car motors and heavy duty diesel oils for trucks to decrease fuel consumption. Although oleic acid was found to perform as a good friction modifier to decrease fuel consumption, it was believed to cause severe corrosion problems with respect to the lead-copper bearings generally used in such engines particularly with respect to the lead component of the bearings. Thereafter, efforts shifted from the acid per se to esters which were found to be less corrosive. U.S. Pat. No. 4,376,056 teaches the use of pentaerythritol oleate in lubricants for spark-ignition and compression-ignition engines to reduce friction and improve fuel economy. United Kingdom published Patent Application GB 2038356A attributes a similar utility to fatty acid esters of glycerol such as glycerol monoleate and glycerol tallowate, see also U.S. Pat. No. 4,304,678 extending this utility to hydroxyl containing acid esters, e.g., glycerol oleate and sorbitan oleate. European Patent Application No. 0 092 946A teaches that the use of lubricants containing both a glycerol ester and certain oil-soluble organic copper compounds improves performance and fuel economy and published U.K. Patent Application 2038355 teaches that improved fuel economy can be obtained using lubricants containing a glycerol ester and zinc O,O-di(2-ethylhexyl)phosphorodithioate.